The invention relates to a circuitry for heartbeat detection, particularly for detecting fetal heartbeat.
A circuit is described in U.S. Pat. No. 4,143,650 for heartbeat detection. The circuit is connected to a transmitter and to a receiver for ultrasound which are placed on the body of a pregnant woman. The ultrasonic signals of a specific carrier frequency emitted by the transmitter are reflected by structures within the body, and the reflected signals which are received by the receiver are converted into electrical signals which are evaluated in the connected circuit. Due to the Doppler effect, ultrasonic signals which are reflected by moving structures, for example, the fetal heart, are frequency shifted relative to the transmitted signals. The frequency-shifted echo signals are demodulated in the connected circuit so that from the velocity of the heartbeat motion a respective Doppler signal results from which the frequency of the fetal heartbeat can be derived.
In this known circuit, various depth ranges can be adjusted by taking into consideration the speed of propagation of the ultrasonic signal within the body of the pregnant woman in such a way that respectively only echo signals originating from this respective depth range are evaluated. In this way, spurious signals originating from moving layers or organs located in a range other than the selected depth range can be suppressed. The depth range is thereby adjusted by the operator according to the presumed depth of the fetal heart.
If the operator does not have any knowledge as to the momentary depth of the fetal heart, the depth range would have to be adjusted in a time consuming and tedious test procedure in which in subsequent search steps varying depth zones would have to be adjusted to finally select the adjustment which in the estimation of the operator would result in the best measuring signal. In addition to the great amount of time required for the adjustment of the correct depth, another disadvantage would be that during the adjustment process the heartbeat signal may be lost if a depth range is selected which does not contain the fetal heart.
Furthermore, due to rather frequently occurring position changes of the fetus, the fetal heart may be moving out of the once selected depth range during the examination so that a dependable measurement of the heartbeat frequency is no longer possible. Since the operator does not have any knowledge as to the direction in which the fetal heart has moved during such a position change, new adjustment of the depth range would in this case also necessitate the tedious test procedure already mentioned.